1. Field of the Invention
The present invention relates to input receivers, and more particularly to a method and apparatus for providing hysteresis for an input receiver to provide a suitable noise margin for an input signal.
2. Description of the Related Art
In earlier integrated circuit (IC) designs, CMOS output drivers were configured as push-pull elements. Consequently, the noise on an output bus fluctuated significantly according to circuit temperature, supply voltage, and fabrication process differences. The noise also varied as a function of the number of devices that were bussed together.
In more recent years, as technological advances have resulted in the scaling of device size and voltage levels, designers have been forced to aggressively address noise problems on external busses in order to maximize the operating speed of circuits within a system. One aspect of more recent output driver solutions has been a move in the industry from push-pull output configurations to differential input receiver configurations. One side of a differential input receiver is supplied with a reference voltage and the other side is driven by an open drain N-channel device. The open drain N-channel devices are typically provided on-chip and bus pull-up terminations are provided either on-chip or externally, such as on a system motherboard or the like.
The aforementioned types of output drivers have become prevalent within the industry. One particular example of this prevalence is exhibited by the Pentium® x86 family of microprocessor products developed by Intel®. Pentium microprocessors employ open drain N-channel output devices to drive a 1.5 Volt (V) bus having a 1.0V reference threshold. More recent bus specifications contemplate lower voltage levels, such as a 1.25V bus having a 0.83V reference threshold. 56 ohm pull-up terminations are typically employed and, while no particular pull down impedance is specified, open drain output drivers are required to comply with bus switching and timing specifications. The industry has adopted the name Assisted Gunning Transceiver Logic (AGTL) to connote a comprehensive set of specifications for devices that interface to this type of bus. These devices are known as AGTL devices or AGTL logic or simply, AGTL.
Conventional input receivers are disadvantageous, however, in situations in which the input signal is noisy. The noisy input signal potentially causes false triggering and erroneous operation among other disadvantageous effects on the IC. For these receivers, their switching or triggering threshold is specified in terms of a voltage range surrounding a designed switching threshold. And the limits of this voltage range are largely established based upon variations in fabrication processes, operating temperature, and operating voltage. Other input devices such as Schmidt Trigger devices have been developed which provide hysteresis for an input signal, but the hysteresis benefits come at a cost of decreased speed, increased power consumption, additional bus loading, etc.
The noise problem is exacerbated by the reduction of voltage according to more recent bus specifications further reducing noise tolerance. It is desired to provide an input receiver that tolerates increased noise levels while maintaining proper logic operation at operative bus voltage levels, including the lower voltage levels associated with newer standards.
It is also desirable to provide an input receiver that can be employed in place of a conventional input receiver to provide increased noise immunity benefits without incurring the penalties associated with present day devices that employ hysteresis such as Schmidt Trigger devices.